Overvoltage protection for series capacitors consists primarily of a large bank of varistors connected in parallel.
Depending on the desired protection levels, temporary overvoltage conditions and the energy absorption capability, the varistors are built up of multiple columns of series-connected ZnO blocks. ABB has the technique and the experience to manufacture such varistors.
The energy impulses have a high-frequency component, leading to increased stress on the blocks and the housings which must be taken into account in the design of the varistors.Varistors may be housed in porcelain or silicone rubber. The former has the advantage of accommodating multiple columns in each housing. The latter has the advantage of flexible and easy erection combined with compactness and low weight in the form of rackassembly. This low weigth alternative has a great advantage when expensive EHV insulated platforms are to be designed for accommodating them.
The MXE racks are pre-assembles at ABB’s works for easy and quick connection at the site.
Varistors with a single-impulse energy up to 140 MJ have been delivered by ABB.